Adjuvant chemotherapy (CTh) is an established postoperative treatment often prescribed to cancer patients to reduce risk of relapse. However, a great proportion of patients report experiencing cognitive, psychomotor, and other functional impairments associated with CTh. Some symptoms may persist for years after treatment and can have dramatic consequences on cancer survivors' quality of life. Despite numerous studies reporting various symptoms during/after CTh treatment, mechanisms underlying these symptoms are not well understood. Recent animal and human brain imaging studies suggest that CTh may have direct neurotoxic effects on the brain, in particular on brain white matter (WM). However, to date, no longitudinal studies have been done to examine the effects of CTh on cortical/subcortical WM integrity in human patients. Furthermore, no effort has yet been made to correlate a given WM structure with its corresponding neurophysiological function (NPF) and behavior. This study proposes longitudinal experiments in women diagnosed with breast cancer to examine the effects of CTh on the corpus callosum (CC) and corticospinal tract (CST) (Aim 1), on NPF that directly depends on the quality of CC/CST structure, and on psychomotor behavior critically relying on CC/CST function (conductivity) (Aim 2). The relationship between CC/CST structural integrity and their corresponding NPF and psychomotor behavior will also be examined (Aim 3). The general underlying hypotheses of the study are that (1) CTh has a direct neurotoxic effect on the CC and CST, resulting in microstructural degeneration and (2) structural damage to the CC and CST impairs relevant neurophysiological function and behavior. This research is expected to yield significant information regarding neural structural and physiological causes of CTh-related side effects; known this information would help more effectively manage the symptoms. By demonstrating direct evidence of detrimental consequences of CTh on the central nervous system, it is hoped that the evidence would encourage pharmaceutical companies to develop safer cancer treatment drugs.